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Fasting for cyclists?

Cyclists consider fasting to lose weight.

Hi Monique,
I really enjoy your column and informative responses. My question revolves around the slippery slope of weight management. A big challenge for me is maintaining lean body mass once the season gets started; when the intensity of the workouts increase while the duration drops off. When the time in the saddle it long and the base miles take me far, it’s relatively easy for me to get to my perceived “race weight.”

I recently went on an endurance ride of four hours, with a couple of tempo efforts thrown in. Unintentionally I did not eat or drink as much as normal, but still had three bottles of a carbohydrate-electrolyte mix and one or two energy bars. What I noticed was that at the end of the ride was that I felt okay and wasn’t even close to bonking.

Would it be possible to cut on the calories consumed during the ride, with the thought that I would tap into my body fat for energy? I’ve coupled the approach with making sure that I have eaten “enough” during the day prior to a ride and it seems like it is working. I have yet to bonk and my weight is slowly decreasing. I realize that this is risky as there is the potential that I end up with a ravenous appetite and eating out of control. Workouts could actually suffer too, as I may not have enough energy to actually do the work.

Thanks for your insight.
A reader

Dear Reader,
What you are really asking about is exercising in the fasted state versus the fed state. Exercising in the fasted state does result in a greater proportion of fat being used for fuel (not a greater number of total calories). During long rides, coaxing your muscles into a higher level of fat burning is appealing for both performance reasons (as this uses less muscle glycogen) and for weight management or becoming lean for the race season.

The fuel mix
First, let’s take a brief look at what fuels you are actually burning when training. We are all keenly aware of our most abundant fuel source of adipose fat, which is thousands of calories of stored energy, even in the leanest cyclist. But newer techniques in fat measurement are unlocking the potential of intramuscular fat or IMTG, fat droplets that resides in your muscle fibers and which can provide 2,000 to 3,000 calories of fuel. Of course the carbohydrate fuels that you burn during training are muscle glycogen and blood glucose. You are always burning both carbohydrate and fat when training. Burning of all these fuels is regulated by exercise intensity.

When you complete endurance training rides for several hours at very low intensities such as 25 to 40 percent VO2 Max, adipose fat releases fat into your bloodstream and is the main fat fuel supply. Increasing intensity to 40 to 65 percent of VO2 max, often called the “fat-burning” zone, keeps the flab fat burning, but also turns on muscle fat burning. At this exercise intensity, about half of energy is supplied from fat, and the other half from carbohydrate. Turning up the intensity to 70 to 80 percent of VO2 max does turn down fat burning a notch, as blood glucose and muscle glycogen can more quickly meet the need for faster fuel.

Hold the carbs?
Currently there is some debate as to the benefits and drawbacks of limiting carbohydrates during low to moderate intensity training sessions. While the data may differ between sedentary subjects, and well trained subjects, let’s talk about trained cyclists.

In regards to pre-exercise fuel, what and how much you eat is often a matter of practicality. For many cyclists early morning training sessions often mean riding out with very little if no pre-exercise fuel. Data does show that consuming carbohydrate before exercise does reduce fat burning during the ride, but this effect lasts only for 60 minutes, after which fat burning levels are similar to when you did have a pre-ride snack. If your goal is to burn more fat during a 60-minute ride, skipping food before training could help. But if you plan to ride longer and need to complete a very specific type of ride successfully, a small carbohydrate snack before a morning ride and consuming a sports drink during the ride provides important fuel and helps you to focus and concentrate on the workout.

You may also be surprised to hear that for a cyclist who has already achieved and maintained a certain level of fitness for this time in the season, that avoiding or limiting carbohydrate during training is not necessarily the best answer to fat burning. While restricting carbohydrate during training can induce adaptations in the muscle to turn up the fat burning, this may not ultimately improve your performance on the bike for racing. There may also be better strategies for getting lean, one of which is a good calorie burning training ride that tips the day’s energy equation to the negative balance.

For example, one study measured the effects of carbohydrate feeding at 30, 60, and 90 minutes intervals and compared the results to cyclists who received no carbohydrate at all during a two-hour ride at 65 percent VO2 max. The carbohydrate feeding did decrease fatty acids available for fuel later in the ride, but ultimately there was no effect on how much fat was used for fuel. What the carbohydrate feedings actually did was maintain blood glucose levels, which can be used as fuel. Muscle glycogen use was not actually affected by the carbohydrate feedings.

Endurance training itself is likely the most important factor in turning up fat burning, particularly muscle fat, and ultimately losing body fat. Weight loss efforts aside, your best performance results are more likely to come from completing training rides at the desired intensity and duration. This also ensures that your energy output is high, ultimately working in favor of losing body fat. You may also want to keep in mind that your approach to turn down carbohydrate intake during training (rather than avoid it completely) may not result in too much of a calorie decrease over the course of a ride. It may just be best to temper your calorie intake later in the day to produce some weight loss results. As you indicated, this may be difficult to do if you finish the ride famished because of efforts to minimize carbohydrate intake while training.

Aside from experiencing fatigue, reduced training pace, and heightened hunger both during and after the ride, longer training sessions with no carbohydrate intake can also place greater stress upon your immune system. Hormones such as cortisol are elevated during and after hard training, and several studies have shown that carbohydrate consumption during endurance training diminishes this increase because normal blood glucose levels are maintained.

Based on this data, I would not encourage you to limit or avoid carbohydrate intake during training. There is no evidence that this will improve your performance for race season. As for weight loss efforts, paying attention to how daily energy intake should be modified for different types of training days (based on intensity and duration) and sticking with a food plan that provides a slight calorie deficit and appropriately controls hunger will bring your closer to reaching and maintaining your body composition goals.

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